But the man did not test positive for the virus itself, the gold standard for infection, and had only mild, cold-like symptoms. He did not require or seek medical attention, according to David Swerdlow, MD, who is leading the CDC's response to the virus.

"We think that this patient was likely infected with MERS," Swerdlow told reporters in a telephone briefing. But he added that the man "technically" is not an official case under the World Health Organization case definition.

That definition requires evidence of the virus in respiratory samples using polymerase chain reaction (PCR) testing. The CDC has traced and tested 60 contacts of the Indiana patient, using PCR, and none has shown evidence of MERS, Swerdlow said.

Secondary blood tests for antibodies are being done to help understand how often the virus is transmitted, Swerdlow said.

The Illinois man is also being retested to confirm the finding, and results should be available soon, Swerdlow said. For the moment, he said, the man has been asked to isolate himself from others, although he is healthy and no longer has any respiratory symptoms.

The Indiana man, who works in a hospital in Saudi Arabia, traveled April 24 from Saudi Arabia to Indiana via plane from London through Chicago and then took a bus to Indiana.

He developed respiratory symptoms April 27, was admitted to hospital April 28, and by May 9 was fully recovered and discharged.

But before being admitted to hospital, the man drove to Illinois and had a 40-minute face-to-face meeting April 25 with his business associate, who has not been further identified. They met again briefly the following day, Swerdlow said.

As part of the investigation of the original case, Swerdlow said, the Illinois man was monitored by the state health department starting May 3 and was tested for MERS on May 5 using PCR.

All 60 contacts of the Indiana man are also being tested for antibodies, but that testing is in its early stages and not all results are available, Swerdlow said.

"This preliminary finding will extend our investigation," he said. "Just as we did with the investigation of the patient in Indiana, we will be contacting, testing, and monitoring people who had close contact with the Illinois resident."

So far, though, nothing in the case has changed the CDC's recommendations for the public, travelers, and healthcare providers, he said.

"For a long time we've known that this virus can be transmitted human to human," he said. "We've just said it doesn't do so easily or does so in a sustained way. I don't think that this significantly changes our understanding."

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